This application aims to further investigate the important issue of how sound energy travels out of the cochlea and to provide a bridge between cochlear mechanics and otoacoustic emissions. In addition to perceiving sound, the ear also makes sound, as a result of the cell-based cochlear nonlinearity. Exactly how this energy radiates from the inner ear has been a major unresolved question since the discovery of these so-called "otoacoustic emissions" in 1976. This application will use the unique and powerful micro-pressure-sensor to probe the cochlear mechanics and a sensitive microphone system as the receiver to calibrate sound and measure the sound pressure in the ear canal. Direct and simultaneous measurements of the intracochlear pressure and ear canal pressures to sound stimuli will be performed in gerbil normal ears in vivo. Cochlear distortion products (DPs) will be used as the ideal intracochlear sound sources of the distortion product otoacoustic emissions (DPOAEs) being detected in the ear canal. The intracochlear approach will be at the basal turn of cochlea, where the cochlear mechanism has been well established in the field and also in the lab. By directly comparing the DPOAEs and DPs, the sound transmission in reverse direction will be further explored with the specific aims to category the role of the reverse traveling wave and the cochlear fluid, the cochlear nonlinearity and the contributions of DPs 'generator' & 'reflector' components to DPOAEs. The measurements will significantly improve our understanding of how the cochlea is excited by and produces sound and how the sound travels out of the cochlea. These sound emissions from the ear are widely used in clinic to detect and diagnose forms of hearing impairment, especially in infants and other patients whose hearing cannot be tested in other ways. Understanding the mechanisms of their generation and transmission is therefore crucial in many clinical and research applications. How exactly sound energy radiates from the inner ear has been a major unresolved question since the discovery of otoacoustic emissions (OAEs) in 1978 by Kemp. In addition to perceiving sound, the cochlea also generates sound, as a result of nonlinear cell-based forces. These 'cochlea-generated' sounds, known as OAEs being detected in the ear canal, are a noninvasive probe used in the clinic to detect and diagnose hearing impairment. Understanding the mechanisms of their generation and transmission is therefore crucial to many clinical and research applications. Recently a simultaneous recording technique of intracochlear pressure and EC pressure in gerbils has been well established in the lab, which has proven to be very illuminating for understanding cochlear mechanics. This proposal aims to use this combined studies of OAEs and intracochlear pressure to probe important aspects of the emission process in normal gerbil cochlea in vivo. The results of this project will lead to further understanding on the specific questions of (1) the intracochlear path OAEs take and (2) the possibility for amplification as these sounds travel out of the cochlea, and (3) the presence of different OAE components. [unreadable] [unreadable] [unreadable] [unreadable]